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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 CdRoSNVlrkBnmgzBihN8pS5Y2UxTSYcs378z/8XCgIEZBmQPB+91AyXB6sbnRqKF
 q2Pri38Zu+vlR7uguDcHAw==

<SEC-DOCUMENT>0001179110-10-013649.txt : 20100910
<SEC-HEADER>0001179110-10-013649.hdr.sgml : 20100910
<ACCEPTANCE-DATETIME>20100910130818
ACCESSION NUMBER:		0001179110-10-013649
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100908
FILED AS OF DATE:		20100910
DATE AS OF CHANGE:		20100910

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Lyons Mark Donald
		CENTRAL INDEX KEY:			0001359049

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-16209
		FILM NUMBER:		101066056

	MAIL ADDRESS:	
		STREET 1:		ONE LIBERTY PLAZA, 53RD FLOOR
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10006

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ARCH CAPITAL GROUP LTD.
		CENTRAL INDEX KEY:			0000947484
		STANDARD INDUSTRIAL CLASSIFICATION:	FIRE, MARINE & CASUALTY INSURANCE [6331]
		IRS NUMBER:				000000000
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		WESSEX HOUSE
		STREET 2:		45 REID STREET
		CITY:			HAMILTON
		STATE:			D0
		ZIP:			HM 12
		BUSINESS PHONE:		441-278-9250

	MAIL ADDRESS:	
		STREET 1:		WESSEX HOUSE
		STREET 2:		45 REID STREET
		CITY:			HAMILTON
		STATE:			D0
		ZIP:			HM 12

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	ARCH CAPITAL GROUP LTD
		DATE OF NAME CHANGE:	20000508

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	RISK CAPITAL HOLDINGS INC
		DATE OF NAME CHANGE:	19950816

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	RISK CAPITAL RE INC
		DATE OF NAME CHANGE:	19950703
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-09-08</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000947484</issuerCik>
        <issuerName>ARCH CAPITAL GROUP LTD.</issuerName>
        <issuerTradingSymbol>ACGL</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001359049</rptOwnerCik>
            <rptOwnerName>Lyons Mark Donald</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>300 PLAZA THREE, 3RD FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>JERSEY CITY</rptOwnerCity>
            <rptOwnerState>NJ</rptOwnerState>
            <rptOwnerZipCode>07311</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman &amp; CEO Arch Insur Gr</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Shares, $.01 par value per share</value>
            </securityTitle>
            <transactionDate>
                <value>2010-09-08</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>2500</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>80.8482</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>52403</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">23,950 of such amount will settle in Issuer's common shares upon the termination of the Reporting Person's employment, as specifically provided in the award agreements.</footnote>
        <footnote id="F2">Represents a weighted average sale price; the sale prices ranged from $80.83 to $80.9018.  Upon request, the full sale information regarding the number of shares sold at each price increment will be provided to the Commission or a security holder of the issuer.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Mark D. Lyons</signatureName>
        <signatureDate>2010-09-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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